body,html {
  width: 100%;
  height: 100%;
  padding: 0;
  margin: 0;
  overflow: hidden;
}
body {
  padding: 10px;
  box-sizing: border-box;
}
#modeBtn {
  font-size: 2rem;
  float: right;
  border: none;
  background: transparent;
}

.night
    background-color: black
    color: white

.light
    background-color white
    color: 